One of known object forming techniques comprises printing a desired pattern with black ink or toner on a heat-expandable sheet that expands and increases in volume when heated, and then uniformly irradiating the entire surface of the heat-expandable sheet with light. This technique takes advantage of the fact that in comparison with the areas where no printing is made, the areas where some printing is made with black ink or toner absorb heat at higher rates and are heated to higher temperatures, and the sheet expands and rises in the areas where some printing is made with black ink or toner. For example, Unexamined Japanese Patent Application Kokai Publication No. 2012-171317 describes an object printing apparatus using the above technique.
On the other hand, a product having a three-dimensional shape formed on a surface of a heat-expandable sheet using the above technique may have the surface erased in part in the course of distribution. For example, if a three-dimensional stamp with an uneven surface is produced using the above technique and the area where numbers and/or characters representing a face value are printed is erased, information regarding the stamp face value is missing and the stamp face value becomes unknown.